The man, who was in his 70s, died from severe thrombocytopenia, a bleeding disorder caused by abnormally low blood platelets, which are needed for blood clotting.

Dr. Tyler Sharp of the U.S. Centers for Disease Control and Prevention's Dengue Branch in San Juan told Reuters the patient had Zika virus disease, which included symptoms of fever, rash and body pain.

Shortly after those symptoms subsided, the man developed "bleeding manifestations" which sent him to the doctor for treatment.

Sharp said the man was diagnosed with a rare Zika complication known as immune thrombocytopenic purpura or ITP, an autoimmune disorder in which the immune system attacks blood cells, called platelets.

Sharp said the ITP case followed the same pattern as patients with Guillain-Barre Syndrome, a paralyzing neurological disorder linked to Zika infections in which the immune system attacks nerves. In both cases, the autoimmune attack occurs after symptoms of Zika have cleared.

Sharp said researchers are studying how Zika causes these rare disorders, and are looking to see whether they are caused by the same mechanism.

“We are actively investigating that. It's very interesting scientifically. But this is something that is a significant cause of morbidity and now mortality here in Puerto Rico, where I live. These are my neighbors. It's of high public health importance that we figure this out and as quickly as we can design some interventions to stop it,” Sharp said.

The death in Puerto Rico is the first U.S. Zika-related death. Previously, Colombia reported three deaths among Zika patients who had symptoms consistent with ITP, Sharp said.

Suriname has also reported one case of Zika-related ITP, and French Polynesia reported four such cases, but all of these patients survived.

Although deaths from Zika are rare, the Puerto Rico death "highlights the possibility of severe cases, as well as the need for continued outreach to raise health care providers' awareness of complications that might lead to severe disease or death," researchers said in a report published in the CDC's Morbidity and Mortality weekly Report.

In addition to the Zika-related death, Puerto Rico reported 683 confirmed cases of Zika, including 65 pregnant women with symptoms of the virus, the CDC said on Friday.

Of the confirmed cases, five patients developed Guillain-Barre syndrome and were hospitalized.

Zika, a virus known to cause the birth defect microcephaly, first began spreading in Puerto Rico in December.

In Brazil, Zika has been linked to 1,198 confirmed cases of microcephaly, a rare birth defect marked by small head size that can lead to severe developmental problems in babies. Zika has also been linked to other severe birth defects and with stillbirth.

The World Health Organization declared Zika a global health emergency on Feb. 1. In addition to microcephaly, the agency says there is a strong scientific consensus that Zika can cause Guillain-Barre, a rare neurological syndrome.

U.S. health officials said Zika remains a public health threat in Puerto Rico, with more cases expected throughout 2016.

Residents of and travelers to Puerto Rico are urged to take steps to avoid mosquito bites including the use of mosquito repellent, take precautions to reduce the risk of sexual transmission of Zika, and seek medical care for any acute illness with rash or fever.